XML 12 R1.htm IDEA: XBRL DOCUMENT v3.25.3
Document And Entity Information - shares
 9 Months Ended
Sep. 30, 2025
Nov. 07, 2025
Document Information [Line Items]    
Entity Central Index Key 0001530249  
Entity Registrant Name FS Bancorp, Inc.  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Period Focus Q3  
Document Fiscal Year Focus 2025  
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2025  
Document Transition Report false  
Entity File Number 001-35589  
Entity Incorporation, State or Country Code WA  
Entity Tax Identification Number 45-4585178  
Entity Address, Address Line One 6920 220th Street SW  
Entity Address, City or Town Mountlake Terrace  
Entity Address, State or Province WA  
Entity Address, Postal Zip Code 98043  
City Area Code 425  
Local Phone Number 771‑5299  
Title of 12(b) Security Common Stock, par value $.01 per share  
Trading Symbol FSBW  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   7,518,462